In a wavelength division multiplexed (WDM) optical network, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined together and transmitted through a series of spans of optical fiber in a WDM fiberoptic network. At a receiver end of a transmission link, the wavelength channels are detected by optical receivers. To that end, the wavelength channels can be separated for individual detection, or they can be detected by coherent receivers having internal oscillators tunable to a wavelength channel of interest.
In a reconfigurable WDM optical network, wavelength channels can be added or dropped at network nodes. From the optical architecture standpoint, it is preferable that any wavelength channel can be added or dropped to any add/drop port, independent of the wavelength of a wavelength channel being dropped or added. To provide this “colorless” add/drop functionality, multicast optical switches (MCS) are used.
Referring to FIG. 1, a prior-art 8×16 MCS 100 includes eight input optical fibers 102, eight optical splitters 104 coupled one-to-one to the eight input optical fibers 102, and sixteen 8×1 selector switches 106 coupled one-to-one to sixteen output optical fibers 108. Each of the optical splitters 104 splits an optical signal supplied by its input optical fiber 102. The split optical signal is coupled to each selector switch 106.
By way of example, an optical signal 110, carrying a plurality of wavelength channels, arrives at e.g. the leftmost optical splitter 104, which then splits the optical signal 110 into sixteen portions 110A, thereby “multicasting” the optical signal 110 to each 8×1 selector switch 106. Due to the multicasting, the optical signal 110 can be dropped at any output optical fiber 108, regardless of the wavelength channel(s) it carries. For example, for the multicast optical signal 110 to be received at the rightmost output fiber 108, the corresponding rightmost 8×1 selector switch 106 selects a corresponding link 112 between the leftmost optical splitter 104 and the rightmost 8×1 selector switch 106 to be coupled to the rightmost output fiber 108. The direction of traveling of the optical signal 110 in the MCS 100 corresponds to a “drop” configuration, but the MCS 100 is optically bidirectional, and for “add” application the optical signal 110 can propagate in the opposite direction.
Since the optical splitting by the optical splitters 104 is “colorless”, that is, not dependent on wavelength, the MCS 100 is “colorless”, that is, its operation does not depend on the wavelength channels being added or dropped. The colorless feature is advantageous over previous prior art systems, in which certain wavelength channels could only be dropped or added at particular locations equipped with corresponding wavelength-selective optical filters. Another important aspect of the MCS 100 is that it is “contentionless”, that is, same wavelength channels may be coupled to multiple common ports of the same MCS 100 for independent routing to different drop ports, without blocking.
Although the colorless and contentionless feature of the prior-art MCS 100 is very useful, a key problem of the MCS 100 is that it is fixed in total port count. The MCS 100 has eight input fibers 102 and sixteen output fibers 108, but in an optical network, the number of directions at a network node varies due to the topology of the network. Therefore, at some locations, many of the input fibers 102 of the MCS 100 would remain unused, resulting in a high initial cost of deployment. Furthermore, if the number of directions grows in the future beyond the number of MCS 100 input optical fibers 102, the blocking will indeed occur. To overcome the blocking problem, costly and service-interrupting re-installation of a larger port count MCS would be required. Due to the extremely high optical data transfer speeds, even a brief service interruption can be very costly to the service provider.